Wind turbine blades generate aerodynamic noise when the wind turbine operates and the rotor is in rotational movement. In general aerodynamic noise source mechanism for lifting air-foils can be classified as follows: a) turbulent boundary-layer flow shed off of the trailing edge, b) separated boundary-layer and stalled airfoil flow, c) vortex shedding due to laminar boundary-layer instabilities, and d) vortex shedding from blunt trailing edges.
It was found out that the main force of aerodynamic noise originates at the trailing edge of the outer span of wind turbine blades. A surprising finding was that noise from the blade tip is of minor importance. It was also found that noise produced by the blades is proportional to the wind speed (i.e. rotational speed) at the blades.
Noise sets a limitation on how close a wind turbine can be built to residential areas and at what rotational frequency a rotor of a wind turbine can rotate.
It is well known to optimise wind turbine blades in the design phase to reduce aerodynamic noise. The attachment of noise reducing devices to the trailing edge of wind turbine blades as a retrofit or after production is well known in the wind turbine industry. One example for a noise reducing device is a serrated plastic plate as a part of a retrofit package attached to the trailing edge of wind turbine blades.
The attachment or production of wind turbine blades with winglets is another means to reduce noise from the wind turbine blade.
EP 1314885 A1 discloses a flexible serrated trailing edge for wind turbine rotor blades. In order to improve the efficiency of an existing wind turbine rotor it is proposed to attach a serrated panel to at least a part of the trailing edge of the wind turbine blades. By using serrated trailing edges the lift and drag characteristics of a lifting surface can be improved.
EP 0652367 A1 proposes a wind turbine having a trailing edge with a saw-tooth form. In order to obtain this form a saw-tooth-shaped strip can be used which is fixed to the trailing edge of the rotor blade.
U.S. 2008/0166241 A1 discloses a wind turbine blade brush with bristles or a brush disposed on an outer surface of the wind turbine blade. The function of the bristles is to achieve a noise reduction effect. The bristles can be arranged in at least one row along a longitudinal direction of the blade or in the vicinity of a trailing edge.
DE 10340978 B1 similarly discloses a wind turbine blade with a brush attached to the trailing edge wherein single fibers of the brush branch out. The branches of the brush imitate feathers of an owl.